The present invention relates to power supplies and more particularly, to the testing, design and fabrication of power supplies so as to prevent arcing.
Arcing is increasingly becoming a serious power supply failure mode mainly because of the ever increasing density of power supply circuitry, achieved by narrower feature spacings and increasing switching frequencies resulting from pressure on power supply designers to keep up with the ever shrinking logic circuitry. Arcing occurs when air gaps with high electric fields becomes electrically conductive (see F. Llewellyn-Jones, xe2x80x9cIonization and Breakdown in Gasesxe2x80x9d, John Wiley and Sons, Inc, 1957). Free electrons present in air, due to natural radioactivity and cosmic events, accelerate under the influence of electric field towards the positively charged feature (anode). At high enough velocities, these free electrons gain enough energy to produce ionizing collisions with air molecules. Each ionizing collision produces an ion and an additional electron that in turn accelerates and causes additional ionizing collisions resulting in electron avalanche. The first effect of this high concentration of primary electrons and ions is a sudden drop in voltage accompanied by glow discharge. Next, the enhanced concentration of electrons, and thus ions, due to the secondary (thermionic and field) emission of electrons from the negatively charged surface (cathode) results in a sharp increase in current and drop in voltage, to a near zero value, across the gap. The secondary emission of electrons from the cathode is the result of positive ions striking the cathode.
In static electric fields, electrons are removed when they reach the anode and are lost to the ionization process. But, if the electric field is periodically reversed, because of switching waveforms, the direction of electron motion will also be periodically reversed. The electrons thus have lesser chance of being lost to the anode, spending more time in the plasma and causing more ionizing collisions and build up of electron and ion concentrations. The electron and ion concentrations can build up to high enough concentrations to cause arcing with less need for the secondary electron emission from the cathode to replace the primary electrons lost to the node. High frequency, thus, aids the avalanche process leading to arcing at lower electric fields.
Ionization and breakdown in gases has been the subject of intense research by physicists, the early work being carried out by J. J. Thomson and J. S. Townsend at the Cavendish Laboratory, Cambridge, U.K. However, the somewhat irreproducible nature of arcing in electronic hardware has discouraged a thorough and comprehensive undertaking by the engineering profession to map out the conditions necessary for arcing in computer power supplies. Generally, an arc damage in field returned power supplies is readily visible to the unaided eye. But in some supplies that fail in the field due to arcing, the damage is to limited in size so as to be invisible to the unaided eye. A possible reason for the limited arc damage is that a fast solid-state circuit breaker extinguishes the arc before it can do any damage to the power supply. As a result, a power supply that trips its fast circuit breaker in the field, due to arcing, may be diagnosed by a failure analysis laboratory as having no defect. It takes painstaking examination of such field failures to identify arcing as the root cause of these failures. There are two relevant papers on this subject. One paper deals with burn down prevention in static power converter equipment (Luderook and M. Ehsani, xe2x80x9cBurndown Prevention in Static Power Converter Equipment:, IEEE Industry Applications Magazine, March/April 1995, pgs. 46-53); and, the other is a military standard for testing power supplies for high altitude applications (MIL-STO-810E, Environmental Testing Methods and Engineering Guidelines, Method 500.3, xe2x80x9cLow Pressure Altitudesxe2x80x9d Sections 11, Jul. 14, 1989).
Therefore it is an object of the present invention to predict the propensity of power supplies to arc in the field.
It is a further object of the present invention to build power supplies with a reduced propensity to arc.
In accordance with the present invention, the power supply to be tested is placed in a vacuum chamber; the power supply is turned on and gas pressure in the chamber is reduced while a camcorder photographs the power supply. As the gas pressure is reduced below 1 atmosphere, the breakdown voltage decreases; when the breakdown voltage decreases to the value of the applied voltage, arcing occurs as long as the applied voltage is greater than a minima. When a site arcs, the site is noted; the power supply is removed from the vacuum chamber; the arc site is conformally coated and the coating cured. Thereafter, the power supply is returned to the vacuum chamber and the test process repeated until the power supply no longer arcs or arcs below a safe pressure considered safe. Thus the partial pressure test can identify the sites that have a possibility of arcing in the field and the design of the power supply modified by use of one or more conformal coatings or by otherwise changing the configuration of the power supply.